In order to improve the strength of a glass article, physical strengthening or chemical strengthening is normally carried out. The chemical strengthening is a treatment in which the glass article is brought into contact with a molten chemical strengthening salt so that an alkali metal element having a relatively small ionic radius in the glass article is ion-exchanged with an alkali metal element having a relatively large ionic radius in the chemical strengthening salt, thereby generating compressive stress on surfaces of the glass article. For example, in the case where alkali metals such as Li, Na, and K are present in the glass article, by replacing the alkali metals in the glass article by alkali metals having larger ionic radii (Li in the glass article is replaced by Na and/or K having a larger ionic radius/radii and Na in the glass article is replaced by K having the larger ionic radius), it is possible to generate compressive stress in a glass surface layer, thereby improving the strength of the glass article and thus enhancing its impact resistance.
In the meantime, it has conventionally been known that while such chemical strengthening is carried out, there arises a problem that, for example, Li ions eluted from the glass article are accumulated in the molten salt and, as the Li-ion concentration in the molten salt gradually increases, the ion exchange is difficult to proceed.
Patent Document 1 (JP-B-S46-39117) discloses that when chemically strengthening a soda-lime glass using a mixed molten salt of KNO3, K2SO4, and KCl, the concentration of Na+ ions, eluted from the soda-lime glass, in the mixed molten salt can be kept low by adding special clay particles, rich in K+ ions and known as hectorite, to the mixed molten salt. Other than the hectorite, Patent Document 1 shows, as a substance for regenerating the chemical strengthening solution, a clay or a clay mineral such as bentonite or montmorillonite, a silicate, a borate, a vitreous or non-vitreous aluminosilicate, a solid gel that can fix protons or alkali metal ions, and so on.